Programmable digital electronic thermostats are known in the art, such as U.S. Pat. No. 4,2O6,872 entitled "Electronic Thermostat" issued June 10, 1980 to Michael R. Levine. A typical thermostat of this type employes a programmable microprocessor which controls the measurement of temperature and the control of a temperature modifying apparatus, such as a heating unit or air conditioning unit, based upon the relationship between the measured ambient temperature and a desired temperature. Typically the desired temperature is selected from a user programmable table of desired temperatures for different times of the day and days of the week. The most simple type of program for the desired temperature is night setback, in which the set point for a heating unit is reduced at night when the occupants are asleep and less heat in needed. Electronic thermostats of this type typically employ a thermistor to measure the temperature.
A thermistor is an electronic component having an electrical resistance which varies with its temperature. Typically such electronic thermostats do not measure the resistance of the thermistor directly. Most typically the resistance of the thermistor is measured by measuring the time constant of an RC circuit in which the resistance of the thermistor is the major contributor to the resistance. This time constant can be measured directly by timing the period needed for the voltage across the capacitor of the RC circuit to charge to a predetermined voltage through the resistance including the thermistor. Alternatively the time constant can be measured indirectly by measuring the frequency of a variable frequency oscillator whose frequency is set by the RC circuit, as disclosed in the above named U.S. Pat. No. 4,2O6,872.
The measurement of the resistance of the thermistor in this manner is advantageous from a cost stand point. The measurement of the time constant of an RC circuit, either directly or via measurement of the frequency of a variable frequency oscillator, can be performed using simple timing circuits which are adaptable for use with the typical microprocessor employed in such electronic thermostats. By contrast the direct measurement of the resistance of the thermistor would require the use of some sort of analog to digital conversion device which would be most expensive.
There are problems with use of such a technique for measurement of temperature. Firstly, the RC time constant is equally sensitive to any variations in the capacitance. This leads to inaccuracies because the typical inexpensive capacitors employed in these thermostats are neither stable or accurately calibrated. Secondly, the time measured by the RC time constant is also sensitive to the supply voltage. In a typical electonic thermostat of this type the power supply regulation is poor. Thus there is a need for an inexpensive technique to obtain an accurate measure of the resistance of a thermistor that is relatively insensitive to variations in capacitance and supply voltage.